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Bulgaria's centre-right Prime Minister Boyko Borisov has tendered his resignation, after a Russia-friendly Socialist candidate defeated his nominee in the presidential election.
Former air force commander Rumen Radev, a political novice, won Sunday's election with about 59% of the vote.
He wants the EU to drop its sanctions on Russia and seeks closer ties with Moscow to help Bulgaria's struggling economy. It is the EU's poorest state.
Bulgaria used to be in the Soviet bloc.
Mr Borisov had backed the centre-right speaker of parliament, Tsetska Tsacheva, who won just over 35% of the vote.
The Bulgarian president has a big role in defence and foreign policy, but most key decisions are taken by the government. He has the power to send legislation back to parliament.
Bulgaria country profile
Mr Radev's triumph reflected widespread discontent with the government's poor record on tackling corruption and poverty.
His success came on the same day as that of Igor Dodon, a pro-Russian Socialist elected president of Moldova.